Go to VTFEdit, create both the .vtf and .vmt file. Check the .vtf to see if thats the correct image (just in case), now go to "tf/materials" and make a folder called "twistmapper" or any other name , put the .vtf and .vmt file there and open .vmt, replace all the text in the vmt for this:
"LightmappedGeneric"
{
"$basetexture" "twistmapper/filename"
"$decal" 1
"$decalsacle" 0.25
"$traslucent" 1
}
That's the config if you are talking about making a poster like the ones used in HL2 for instance.You can play around the configs and you can edit the "basetexture" line with the folder you want the files to be on, just remember to replace "filename" for the name of your vtf and vmt, you don't need to write down the extension in the vmt.
If everything is ok, you should be able to see the new texture in the texture tab. In order to place it as a decal you need to select the texture and use the "Apply overlays" tool and remember to have Texture Lock selected in order to change the size of the decal.
I hope that helped, and I hope I understood what you tried to do.